docs: def knit(self, input_file, opts_chunk='eval=FALSE'):
          """Use Knitr to convert the r-markdown input_file
          into markdown, returning a file object.
          """
          # use temporary files at both ends to allow stdin / stdout
          tmp_in = tempfile.NamedTemporaryFile(mode='w+')
          tmp_out = tempfile.NamedTemporaryFile(mode='w+')
  
          tmp_in.file.write(input_file.read())
          tmp_in.file.flush()
          tmp_in.file.seek(0)
  
          self._knit(tmp_in.name, tmp_out.name, opts_chunk)
          tmp_out.file.flush()
          return tmp_out